Fix 50-firmware.rules no such file on Fedora 19

systemd on Fedora 19 has made /lib/udev/rules.d/50-firmware.rules
optional:
http://lists.freedesktop.org/archives/systemd-devel/2013-March/009708.html

Firmware is now handled by the in-kernel loader.

Added a file exist check for Fedora 19 and any other distros that
may have dropped the rules file. Backwards compatible with existing
distros.

Fixes: bug #1196409

Change-Id: I1d2acf3f88543736d75eb2e8766e83a3da194638